The present invention relates to a drilling assembly wherein a drill bit, a bit drive motor and resistivity measurement while drilling instrument are carried on the end of a drill string made of casing and are retrievable through the casing which may be left in the wellbore to serve as the permanent casing or wellbore liner.
Conventional rotary drilling operations require relatively frequent withdrawal of the conventional drill string from the wellbore to inspect or replace the drill bit or portions of the drill string, to perform well logging operations and to install permanent well casing. This insertion and withdrawal process is time consuming, hazardous to the operating personnel and increases the possibility of damaging the well due to inadvertent dropping of the drill string into the wellbore or encountering the influx of formation fluids into the wellbore due to the swapping effect encountered during the drill string insertion and removal process.
Various efforts have been made to limit the need to insert and withdraw the drill string for well logging purposes. For example, Meader, et al U.S. Pat. No. 4,785,247 issued on Nov. 15, 1988, provides an electromagnetic resistivity measurement system built into the drill string itself to allow measurement of formation resistivity during the drilling process. Gearhart, et al U.S. Pat. No. 3,964,556 issued on Jun. 22, 1976 provides a system for transmitting information, such as the resistivity measurement generated by the Meador system, from the lower end of a drill string through the drilling fluid to equipment at the surface location of the borehole. While the Gearhart patent suggests that transmission of information through the drilling fluids should occur only at a time when actual drilling has ceased, later developments have allowed continuous transmission of logging while drilling information. See, for example, the publication entitled, "Development and Successful Testing of a Continuous-Wave, Logging-While-Drilling Telemetry Systems" by B. J. Patton, et al. pages 1215-1221, Journal of Petroleum Technology, Oct. 1977.